# Thread: how to use dgeev.c

1. ## how to use dgeev.c

can anyone simplify the arguments for the following routine in dgeev.c for me? I am having trouble figuring out how to use this properly.

Code:
```int dgeev_(char *jobvl, char *jobvr, integer *n, doublereal *a,  integer *lda, doublereal *wr, doublereal *wi, doublereal *vl,
integer *ldvl, doublereal *vr, integer *ldvr, doublereal *work,
integer *lwork, integer *info)```
Arguments
=========

JOBVL (input) CHARACTER*1
= 'N': left eigenvectors of A are not computed;
= 'V': left eigenvectors of A are computed.

JOBVR (input) CHARACTER*1
= 'N': right eigenvectors of A are not computed;
= 'V': right eigenvectors of A are computed.

N (input) INTEGER
The order of the matrix A. N >= 0.

A (input/output) DOUBLE PRECISION array, dimension (LDA,N)
On entry, the N-by-N matrix A.
On exit, A has been overwritten.

LDA (input) INTEGER
The leading dimension of the array A. LDA >= max(1,N).

WR (output) DOUBLE PRECISION array, dimension (N)
WI (output) DOUBLE PRECISION array, dimension (N)
WR and WI contain the real and imaginary parts,
respectively, of the computed eigenvalues. Complex
conjugate pairs of eigenvalues appear consecutively
with the eigenvalue having the positive imaginary part
first.

VL (output) DOUBLE PRECISION array, dimension (LDVL,N)
If JOBVL = 'V', the left eigenvectors u(j) are stored one
after another in the columns of VL, in the same order
as their eigenvalues.
If JOBVL = 'N', VL is not referenced.
If the j-th eigenvalue is real, then u(j) = VL(:,j),
the j-th column of VL.
If the j-th and (j+1)-st eigenvalues form a complex
conjugate pair, then u(j) = VL(:,j) + i*VL(:,j+1) and
u(j+1) = VL(:,j) - i*VL(:,j+1).

LDVL (input) INTEGER
The leading dimension of the array VL. LDVL >= 1; if
JOBVL = 'V', LDVL >= N.

VR (output) DOUBLE PRECISION array, dimension (LDVR,N)
If JOBVR = 'V', the right eigenvectors v(j) are stored one
after another in the columns of VR, in the same order
as their eigenvalues.
If JOBVR = 'N', VR is not referenced.
If the j-th eigenvalue is real, then v(j) = VR(:,j),
the j-th column of VR.
If the j-th and (j+1)-st eigenvalues form a complex
conjugate pair, then v(j) = VR(:,j) + i*VR(:,j+1) and
v(j+1) = VR(:,j) - i*VR(:,j+1).

LDVR (input) INTEGER
The leading dimension of the array VR. LDVR >= 1; if
JOBVR = 'V', LDVR >= N.

WORK (workspace/output) DOUBLE PRECISION array, dimension (LWORK)
On exit, if INFO = 0, WORK(1) returns the optimal LWORK.

LWORK (input) INTEGER
The dimension of the array WORK. LWORK >= max(1,3*N), and
if JOBVL = 'V' or JOBVR = 'V', LWORK >= 4*N. For good
performance, LWORK must generally be larger.

If LWORK = -1, then a workspace query is assumed; the routine
only calculates the optimal size of the WORK array, returns
this value as the first entry of the WORK array, and no error
message related to LWORK is issued by XERBLA.

INFO (output) INTEGER
= 0: successful exit
< 0: if INFO = -i, the i-th argument had an illegal value.
> 0: if INFO = i, the QR algorithm failed to compute all the
eigenvalues, and no eigenvectors have been computed;
elements i+1:N of WR and WI contain eigenvalues which
have converged.

2. Do you know what a pointer is? How much of it have you managed to decipher?

3. i do know what a pointer is, and i know what to put for n, a and lda. i am not sure about what to put for the others and Y. I will only be using this on a nxn matrix.

jason

4. can anyone tell me how to resolve these errors?

hw5.c:41: warning: passing argument 3 of 'dgeev_' from incompatible pointer type
hw5.c:41: warning: passing argument 5 of 'dgeev_' from incompatible pointer type
hw5.c:41: warning: passing argument 9 of 'dgeev_' from incompatible pointer type
hw5.c:41: warning: passing argument 11 of 'dgeev_' from incompatible pointer type
hw5.c:41: warning: passing argument 13 of 'dgeev_' from incompatible pointer type
hw5.c:41: warning: passing argument 14 of 'dgeev_' from incompatible pointer type

here is the relevant piece of code.

Code:
```double **A;
double *W;
double *X;
double *p;
double **y;
double **z;
double set=0.0;
int i,j,k;
int n=6;
int m=n*6;
A=matrix(n,n);
y=matrix(n,n);
z=matrix(n,n);
W=vector(n);
X=vector(n);
p=vector(m);
dgeev_(&d, &d, &n, (double *)A, &n, W, X, (double *)y, &n, (double *)z, &n, p, &m, &i);```
thanks for any help

5. It would seem that int * is not compatible with "integer *". What is "integer" defined as? It's not a C type, so it presumably has been typedef'd elsewhere.

6. this was originally a fortran routine that was converted to be made compatible with C.
one of my header files is f2c.h